1. Chapter 4 - Visual Basic Schneider
General Procedures
Chapter 4 - Visual Basic Schneider

2. Chapter 4 - Visual Basic Schneider
Outline & Objective
Sub Procedures
Procedure Parameters
Function Procedures
Modular Design
Chapter 4 - Visual Basic Schneider

3. Chapter 4 - Visual Basic Schneider
What is a procedure?
A general procedure is a set of commands that is given a name so that it can be invoked by another part of the program
Procedures make a program easier to develop, test, and correct
Code written to perform one well-defined subtask is referred to as a Sub procedure or function
A Sub procedure is a part of a program that performs one or more related tasks, has its own name,
To distinguish them from event procedures, Sub procedures and functions are referred to as a General Procedures
Sub procedures allow us to focus on the tasks and later on how to accomplish each task
Chapter 4 - Visual Basic Schneider

4. Chapter 4 - Visual Basic Schneider
General Procedures
In Visual Basic, there are two types of procedures:
Sub
Function
Note: To distinguish procedures from event procedures, Sub and Function procedures are referred to as general procedures.
As a rule, a Sub procedure should perform only one task, or several closely related tasks, and should be kept relatively small.
Chapter 4 - Visual Basic Schneider

5. Sub Procedures Properties:
may be called
may be passed data called arguments
may return values to the calling program
may change the data stored in a received variable
Chapter 4 - Visual Basic Schneider

6. Passing Arguments to Subs:
When you define a Sub procedure, sometimes you need to transfer variables that are used in different Subs. This is called passing in programming languages.
Chapter 4 - Visual Basic Schneider

7. Components of Sub Procedure:
name: used to identify the Sub procedure
parameters: a Sub procedure accepts values from the caller through its parameters. It may also send values back to the caller through its parameters.
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8. Chapter 4 - Visual Basic Schneider
Sub Procedure's Name
The rules for naming Sub Procedures are the same as naming variables.
In this text, Sub procedure names begin with uppercase letters in order to distinguish them from variable names.
Chapter 4 - Visual Basic Schneider

9. Syntax of a Sub Procedure
Private Sub ProcedureName ( )
statement(s)
End Sub
Chapter 4 - Visual Basic Schneider

10. Creating Visual Basic Sub Procedure:
Activate the code window
Select Add Procedure from the Tools menu
Type in the name of the Sub procedure
Click on Private in the Scope box
Press the Enter key or click the OK button
Type the statements of the Sub procedure into the code window
Chapter 4 - Visual Basic Schneider

11. Example of Call to a Sub Procedure:
Private Sub cmdCompute_Click()
Dim num As Single
num = Val(InputBox("Enter a number:"))
Call Triple(num)
End Sub
softDrinks = Val(txtDrinks.Text)
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12. Chapter 4 - Visual Basic Schneider
Sub Procedure Triple:
Private Sub Triple(num As Single)
' Multiply the value of the number by 3
picResult.Print "The number is"; 3 * num
End Sub
Chapter 4 - Visual Basic Schneider

13. Passing Arguments to Sub Procedures
Arguments : Variables or expressions placed in parentheses in a Call statement.
Not only is the value of the argument passed to the parameter, but the value of the parameter is passed back to the argument.
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14. Chapter 4 - Visual Basic Schneider
Example of Arguments
Call Triple(num)
Example 5.4.1
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15. Chapter 4 - Visual Basic Schneider
Parameters
Variables placed in parentheses after a Sub Procedure's name.
When the procedure is called, the values of the corresponding arguments are placed in the parameters.
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16. Chapter 4 - Visual Basic Schneider
Example of Parameters
Private Sub Triple(num As Single)
Chapter 4 - Visual Basic Schneider

17. Passing arguments to parameters
Call Triple(num)
Private Sub Triple (num As Single)
Argument
Go over example 4.1.6
The second CALL statement uses different variable names for the arguments to show that using the same argument names is not necessary.
Parameter
Chapter 4 - Visual Basic Schneider

18. Passing Arguments to Parameters
Call Add (x, y )
Private Sub Add (num1 As Single, num2 As Single)
Arguments
Go over example 4.1.6
The second CALL statement uses different variable names for the arguments to show that using the same argument names is not necessary.
Parameters
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19. Chapter 4 - Visual Basic Schneider
Passing Arguments
A Sub procedure receives the location of the arguments, and may use and modify the value of the arguments stored at that location.
Two-way into and out of the Sub Procedure.
Example 1.4.2
It provides a vehicle for passing values from a Sub procedure back to the place which the Sub procedure was called
Different names may be used for an arguments and its corresponding parameter
Only one memory location is involved
Example 2.4.2
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20. Chapter 4 - Visual Basic Schneider
Passing Arguments
Private Sub cmdDisplay_Click()
Dim amt As Single
amt = 2
picResults.Print amt;
Call Triple(amt)
picResults.Print amt
End Sub
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21. Chapter 4 - Visual Basic Schneider
Sub Triple
Private Sub Triple(num As Single)
' Triple a number
picResults.Print num;
num = 3 * num
End Sub
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22. Chapter 4 - Visual Basic Schneider
Passing Arguments
Call Triple(amt)
Private Sub Triple (num As Single)
amt
num
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23. Passing Data - by Reference
amt
amt
amt 2 2 6 6
num
num
Chapter 4 - Visual Basic Schneider

24. Important Rules for Passing Arguments to a Sub
The number of arguments and parameters must match.
The data type of each argument must match its corresponding parameter.
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25. Chapter 4 - Visual Basic Schneider
Local Variables:
A variable that is used only in a specific procedure (Sub or Function).
The scope of the local variable is the Sub or Function in which that variable has been defined.
When the same variable name appears in two different Sub procedure, Visual Basic gives the variables a separate identity and treats them as two different variables.
Also each time a Sub procedure is called, all declared variables that are not parameters assume their default values.
Example (the value of num is set to default value)
Example (x is treated as different variable)
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26. Chapter 4 - Visual Basic Schneider
Local Variables:
Declared within a procedure definition
Private to a procedure definition
Variables in different procedures are totally independent
Different procedures can have variables with the same names; however, each variable will have its own memory location
Chapter 4 - Visual Basic Schneider

27. Advantages of Local Variables
Extremely useful for team programming
Protects against accidental side effects (change of the value of the variable in one procedure that causes an error in another)
Chapter 4 - Visual Basic Schneider

28. Example of Local Variables
Private Sub cmdButton_Click()
Dim var1 As Integer, var2 As Integer, num As Integer
var1 = 2;
var2 = 4;
Call Add(num);
picBox.Print num;
End Sub
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29. Chapter 4 - Visual Basic Schneider
Sub Add
Private Sub Add(num As Integer)
Dim var1 As Integer, var2 As Integer
num = var1 + var2
End Sub
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30. Chapter 4 - Visual Basic Schneider
Form-Level Variables
Form-level variables are visible to every procedure.
Form-level variables appear at the top of the code window.
Making a variable visible to every procedure in a form’s code without being passed.
When a form-level variable is assigned a value by a procedure, it retains its value when the procedure is exited
Chapter 4 - Visual Basic Schneider

31. How to create Form-Level Variables?
Activate the code window
Click on the down arrow to the right of the object list box
Click on General
Click on Declaration in the procedure list box
Type in Dim statements for form-level variables
Example 4.2.6
Delete Dim statements in Declaration section of General and show the resut
The sum of 2 and 3 is 5
num1 = 2
num2 = 3
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32. Chapter 4 - Visual Basic Schneider
Example
' In Declaration section of General
Dim num1 As Single, num2 As Single
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33. Chapter 4 - Visual Basic Schneider
Common Errors
Passing incorrect data types.
Not returning the result of the computation back to the calling program.
The only way that a Sub procedure can return a value is through its argumnets
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34. Chapter 4 - Visual Basic Schneider
Another Example
Private Sub cmdDisplay_Click()
' Demonstrate that variables in a Sub procedure do
' not retain their values in subsequent calls
Call Three
End Sub
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35. Chapter 4 - Visual Basic Schneider
Sub Three
Private Sub Three()
Dim num As Single
'Display the value of num and assign it the value 3
picResults.Print num;
num = 3
End Sub
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36. Chapter 4 - Visual Basic Schneider
What is a function?
Similar to a Sub, performs a specific task
Unlike a Sub, returns a single value to the calling program
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37. Chapter 4 - Visual Basic Schneider
Types of Functions
Standard functions (built-in)
User-defined functions
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38. User-Defined Function
A function returns a single value
The value is returned by assigning a value to the name of the function
The values of the parameters of a function should not be used for returning values from a function
It returns a single value
This value is returned in the function itself, not as an argument
A function is called by using its name in an expression
the arguments of a function should not be altered in the function
If a procedure needs to return more than one value, a SUB procedure should be used instead.
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39. Chapter 4 - Visual Basic Schneider
The Function Syntax
Private Function FunctionName(parameter-list) As dataType
Statement(s)……
…..
FunctionName = value
End Function
Note: value must be of the type dataType specified in the function declaration
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40. Chapter 4 - Visual Basic Schneider
Example of a Function (using a function to change from Fahrenheit to Celsius)
Private Sub cmdConvert_Click()
picTempC.Cls
picTempC.Print FtoC(Val(txtTempF.Text))
End Sub
Private Function FtoC(t As Single) As Single
‘ Convert Fahrenheit temperature to Celsius
FtoC = (5 / 9) * (t - 32)
End Function
Chapter 4 - Visual Basic Schneider

41. Rules for Defining and Calling a Function
User-defined function must include a statement that assigns the function name a value.
User-defined functions are called in the same way that built-in functions are called.
A user-defined function may be called in an expression.
Chapter 4 - Visual Basic Schneider

42. A function can receive many values
Returning Value
A function can receive many values
Only one value can be directly returned
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43. Chapter 4 - Visual Basic Schneider
Example of a Function
Private Sub cmdDetermine_Click()
Dim nom As String
nom = FirstName(txtFullName.Text)
picFirstName.Print "The first name is “; nom
End Sub
Private Function FirstName(nom As String) As String
Dim firstSpace As Integer
firstSpace = InStr(nom, " ")
FirstName = Left(nom, firstSpace - 1)
End Function
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44. Chapter 4 - Visual Basic Schneider
Common Errors
Passing incorrect data types
Not specifying the data type of the returned value
Not assigning a value to the Function name inside the Function definition
Misspelling of the Function name
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45. Chapter 4 - Visual Basic Schneider
Further Examples
Private Sub cmdDisplay_Click()
Call DisplayVolume(1, 2)
Call DisplayVolume(3, 4)
End Sub
Private Sub DisplayVolume(r As Single, h As Single)
PicOutput.Print "Volume of cylinder with base area "; Area(r)
PicOutput.Print "and height"; h; "is"; h * Area(r)
Private Function Area(r As Single) As Single
Area = * r ^ 2
End Function
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46. Chapter 4 - Visual Basic Schneider
Another Example
Private Sub cmdDisplay_Click()
Dim a As String
a = “Choo ”
picOutput.Print = TypeOfTrain()
End Sub
Private Function TypeOfTrain() As String
Dim a As String
a = a & a
TypeOfTrain = a & “train”
End Function
Output: Train
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47. Chapter 4 - Visual Basic Schneider
Last Example
Private Sub cmdDisplay_Click()
Dim num As Single
num = 5
picOutput.Print Triple (num)
picOutput.Print num
End Sub
Private Function Triple(x As Single) As Single
num = 3
Triple = num * x
End Function
Output: 15 5
Chapter 4 - Visual Basic Schneider